In modern telecommunications networks, traffic needs to be protected against failures in order to give very high service availability.
Transmission resources are accordingly redundant and back-up resources are provided in addition to service resources. A failure is recovered in a very short time by routing traffic from the faulty service resource to a back-up resource.
Routing of traffic is often made by means of physical switches, electronic or even electro-mechanic, which disconnect the faulty resource and connect the back-up one.
In some cases there are no physical switches and the routing of traffic from the faulty resource to the back-up resource is done by a simultaneous turning on/turning off of transceiver of the back-up/backed-up resource. Such simultaneous switching is referred to as virtual switch.
Examples of virtual switches can be found in radio equipment protection where the air interface is common to service and back-up radio equipment. Other virtual switches can be found in protection equipment of electrical and optical interfaces based on external power splitters and combiners. These elements are provided externally to the equipment in order to maximize the fault coverage, placing the switch element at the extreme edge of the unit to protect. As in radio protection, in these cases service and back-up resources share the same physical medium, i.e. the same electrical or optical line.
A key aspect of protection schemes is robustness, namely, the ability of the protection scheme to accomplish the protection switching action in any failure scenario. Robustness can be impaired by a number of reasons, among which there is the incomplete control of the protection switch because, for instance, of the unavailability of controllers involved in the protection operations.
Once the switching action is performed by means of physical switches, unavailability of the control chain is not a big issue because a physical switch features only two states so that service is assigned in a mutually exclusive way to only one of the two resources.
The situation is completely different in case of the switch is a virtual switch rather than a physical switch.
FIG. 1 depicts a known protection group based on virtual switches, where only one working resource 1 to be backed-up and one back-up resource 2 are shown. In the figure, the two resources 1 and 2 are connected at one end to a same communication medium (e.g. air, an optical fiber, or an electrical cable) as if a virtual switch 3 were present for switching traffic to/from the active resource 1 or 2, while at the other end a physical switch 4 is provided, which can be physically operated for switching traffic from/to the active resource 1 or 2.
This configuration is a typical one: virtual switches are usually present only at one side of the resource, while physical switches are at the other side, normally inside the equipment.
In order to implement a virtual switch 3, each of the resources 1 and 2 comprises a controller for switching on or switching off the corresponding transceiver or other active elements of the resource and, accordingly, enabling or disabling the entire resource to reception/transmission of telecommunications data.
The transceiver or the active elements of the resource are set active, in a nearly static way, by turning on the transceiver, while the protection is set inactive by setting off the transceiver.
Once a protection switching is requested, the controllers take care of switching the traffic by reversing the setting of both transceivers. Occurrence of a protection switching is a very rare event so that setting is assumed to be nearly static.
A virtual switch is the result of two coordinated, but distinct, actions: turning off active elements in the faulty resource 1 and turning on active elements in the back-up resource 2.
Failure of one of this actions leads to an inconsistent status, both resource active or inactive, with subsequent service disruption. An inconsistent status needs an intervention by an operator to manually recover proper operation.
As already mentioned above, these inconsistent scenarios may happen in case of unavailability of one of the controllers just because of the failure. Even a communication failure between controllers can lead to inconsistent status of the virtual switch. The scenario can be further complicated when a plurality of controllers are present as it happens in complex protection schemes.